1. Field of the Invention
This invention relates to an electron source, an image forming apparatus which is an application thereof, and a method of manufacturing the electron source.
2. Description of the Related Art
Two types of electron sources, namely thermionic sources and cold cathode electron sources, are known as electron-emitting devices. Examples of cold cathode electron sources are electron-emitting devices of the field emission type (abbreviated to "FE" below), metal/insulator/metal type (abbreviated to "MIM" below) and surface-conduction emission type (abbreviated to "SCE". Known examples of the FE type are described by W. P. Dyke and W. W. Dolan, "Field emission", Advance in Electron Physics, 8,89 (1956) and by C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47,5248 (1976).
A known example of the MIM type is described by C. A. Mead, "The tunnel-emission amplifier", J. Appl. Phys., 32,616 (1961).
A known example of the SCE type is described by M. I. Elinson, Radio. Eng. Electron Phys., 10 (1965).
The SCE type makes use of a phenomenon in which an electron emission is produced in a small-area thin film, which has been formed on a substrate, by passing a current parallel to the film surface.
Various examples of this surface-conduction electron-emitting device have been reported. One relies upon a thin film of SnO.sub.2 according to Elinson, mentioned above. Other examples use a thin film of Au [G. Dittmer: "Thin Solid Films", 9,317 (1972)]; a thin film of In.sub.2 O.sub.3 /SnO.sub.2 (M. Hartwell and C. G. Fonstad: "IEEE Trans. E. D. Conf.", 519 (1975); and a thin film of carbon (Hisashi Araki, et al: "Vacuum", Vol. 26 No. 1, p. 22 (1983).
FIG. 1 illustrates the construction of the device according to M. Hartwell, described above. This device is typical of the surface-conduction electron-emitting device. As shown in FIG. 1, numeral 1 denotes an insulative substrate. Numeral 2 denotes a thin film for forming an electron emission portion. The thin film 2 comprises a thin film of a metal oxide formed into an H-shaped pattern by sputtering. An electron emission portion 3 is formed by an electrification process referred to as "forming", described below. Numeral 4 designates a thin film, which includes the electron emission portion 3. Further, spacing L1 between device electrodes is set to 0.5.about.1 mm, and W is set to 0.1 mm. It should be noted-that since the position and shape of the electron emission portion 3 is unknown, this is represented schematically.
In these conventional surface-conduction electron-emitting devices, generally the electron emission portion 3 is formed on the thin film 2, which is for forming the electron emission portion, by the so-called "forming" electrification process before electron emission is performed. According to the forming process, a DC voltage or a very slowing rising voltage (e.g., on the order of 1 V/min) is impressed across the thin film 2, which is for forming the electron emission portion, thereby locally destroying, deforming or changing the property of the thin film 2 and forming the electron emission portion 3, the electrical resistance of which is high. The electron emission portion 3 causes a fissure in part of the thin film 2, which is for forming the electron emission portion. Electrons are emitted from the vicinity of the fissures. The thin film 2 for forming the electron emission portion inclusive of the electron emission portion produced by forming shall be referred to as the thin film 4 inclusive of the electron emission portion. In the surface-conduction electron-emitting device that has been subjected to the above-described forming treatment, a voltage is applied to the thin film 4 inclusive of the electron emission portion, and a current is passed through the device, whereby electrons are emitted from the electron emission portion 3. Various problems in terms of practical application are encountered in these conventional surface-conduction electron-emitting devices. However, the applicant has solved these practical problems by exhaustive research regarding improvements set forth below.
Since the foregoing surface-conduction electron-emitting device is simple in structure and easy to manufacture, an advantage is that a large number of devices can be arrayed over a large surface area. Accordingly, a variety of applications that exploit this feature have been studied. For example, electron beam sources and display apparatuses can be mentioned. As an example of a apparatus in which a number of surface-conduction electron-emitting devices are formed in an array, mention can be made of an electron source in which surface-conduction electron-emitting devices are arrayed in parallel (referred to as a "ladder-shaped" array) and both ends of the individual devices are connected by wiring (also referred to as common wiring) to obtain a row, a number of which are provided in an array (for example, see Japanese Patent Application Laid-Open NO. 1-031332, filed by the applicant). Further, in an image forming apparatus such as a display apparatus, flat-type displays using liquid crystal have recently become popular as a substitute for CRTs. However, since such displays do not emit their own light, a problem encountered is that they require back-lighting. Thus, there is a need to develop a display apparatus of the type that emits its own light. An image forming apparatus that is a display apparatus comprising a combination of an electron source, which is an array of a number of the surface-conduction electron-emitting devices, and phosphors that produce visible light in response to the electrons emitted by the electron source is comparatively easy to manufacture, even as an apparatus having a large screen. This apparatus is a display apparatus capable of emitting its own light and has an excellent display quality (for example, see U.S. Pat. No. 5,066,883, issued to the applicant).
However, the following problems are encountered in the above-described electron source having a number of the surface-conduction electron-emitting devices arrayed on a substrate, in the method of manufacturing an image forming apparatus using the electron source, and particularly in the aforesaid forming process:
In the image forming apparatus, the number of electron-emitting devices needed to obtain high-quality image or picture is very large. In the forming process used when manufacturing the electron-emitting devices, a plurality of the surface-conduction electron-emitting devices are connected, and the current that flows through the wiring (the aforementioned common wiring), which supplies power to each device from an external power supply, becomes large. This gives rise to the following shortcomings:
1) Owing to a voltage drop produced by the resistance of the common wiring, the voltage applied to each device develops a gradient and therefore a disparity occurs in the voltage applied to the devices in the forming process. As a consequence, the electron emission portions formed also change and the device characteristics become non-uniform. PA1 2) Since the forming process is carried out by electrification, namely by passing electric current, using the common wiring, power in the wiring due to electrification is expended as heat, and a temperature distribution is produced on the substrate. This impresses a distribution upon the device temperature of each portion and the electron emission portions formed also undergo a change. A variance in characteristics from one device to another thus tends to occur. PA1 3) Since formation of the electron emission portions is carried out by passing of current using the wiring, power in the wiring due to electrification is expended as heat, the substrate experiences heat damage and strength against impact declines. PA1 (1) The number of devices capable of being connected by common wiring is essentially limited. PA1 (2) In order to reduce wiring resistance, it is necessary to use comparatively expensive materials such as gold or silver. This raises expenditures for raw materials. PA1 (3) In order-to reduce wiring resistance, it is required that thick wiring electrodes be formed. This lengthens the time required for the manufacturing process, namely the formation of the electrodes and patterning, and raises the cost of the related equipment and facilities.
Though these problems have been described in the case of the ladder-shaped arrangement of the plurality of electron-emitting devices on the substrate, similar problems occur as set forth below also in the case of a simple matrix arrangement, described later.
Problem 1) mentioned above will be described in further detail with reference to FIGS. 3A, B, C and FIGS. 4A, B, C. In both of these diagrams, A is an equivalent circuit diagram which includes electron-emitting devices, wiring resistors and a power supply, B is a diagram illustrating potential on high- and low-potential sides of each device, and C is a diagram showing a difference voltage, namely applied device voltage, between the high- and low-potential sides of each device.
FIG. 3A illustrates a circuit in which N-number of parallel-connected electron-emitting devices D.sub.1 .about.D.sub.N and a power supply VE are connected through wiring terminals TH, TL. The power supply and device D.sub.1 are connected, and the ground side of the power supply is connected to the device D.sub.N. The common wiring connecting the devices in parallel includes resistance components r between mutually adjacent devices, as illustrated. (In an image forming apparatus, pixels that are the targets of electron beams usually are arrayed at an even pitch. Accordingly, the electron-emitting devices also are arrayed so as to be evenly spaced apart. The wiring connecting the devices has approximately equal resistance values between the devices as long as width and film thickness do not develop variance in terms of manufacture.)
Further, the electron-emitting devices D.sub.1 .about.D.sub.N are assumed to have approximately equal resistance values of Rd.
In case of a circuit of the kind shown in FIG. 3A, a voltage which is greater closer to the two end devices (D.sub.1 and D.sub.N) is applied, as evident from FIG. 3C, with the applied voltage being lowest at the devices in the vicinity of the center.
FIGS. 4A, B, C are for a case in which the positive and negative electrodes of the power supply are connected to one side [the side of device D.sub.1 in FIG. 4A] of the array of parallel-connected devices. The voltage applied to each device is greater closer to the device D.sub.1, as illustrated in FIG. 4C.
The degree of variance in the applied voltage from one device to another as indicated in the two above-described examples differs depending upon the total number N of parallel-connected devices, the ratio (=Rd/r) of the device resistance Rd to the wiring resistance r or the position at which the power supply is connected. In general, however, variance becomes more prominent the larger the value of N and the smaller the value of Rd/r. Further, the method of connection in FIGS. 4A, B, C results in greater variance in the voltage applied to the devices than the method of connection shown in FIGS. 3A, B, C. Furthermore, though the arrangement is different from those of the two above-described example, simple matrix wiring of the kind illustrated in FIG. 5 also develops a variance in terms of the applied voltage of each device owing to a voltage drop that occurs across wiring resistors r.sub.x and r.sub.y. In a case where a plurality of devices are connected by common wiring, the applied voltage of each device develops a variance unless the wiring resistance is made sufficiently small in comparison with the device resistance Rd.
The inventors have discovered the following facts as a result of intensive research: Specifically, in a case where forming is carried out in the process of forming an electron emission portion of an electron-emitting device, forming is performed at the same voltage or power if the shape of device is the same, i.e., if the material and film thickness of the thin film 2 for forming the electron emission portion of FIG. 1, as well as W, L, are the same. The voltage or power specified to the device is referred to as device forming voltage V.sub.form or P.sub.form, respectively. When it is attempted to carry out the forming process by applying a voltage or power much higher than V.sub.form or P.sub.form to an device, the electron emission portion of the device undergoes a great change in form and the electron emission characteristic deteriorates. If the applied voltage or power is less than V.sub.form or P.sub.form, it goes without saying that the electron emission portion cannot be formed.
On the other hand, in a case where a plurality of devices connected by common wiring are formed simultaneously by supply of voltage through the common wiring from an external power supply, a disparity in the voltage applied to each device occurs owing to a voltage drop in the wiring, and devices are produced in which the voltage or power applied thereto exceeds the forming voltage V.sub.form or forming power P.sub.form. It known qualitatively that the electron emission portions of these devices deteriorate and that the electron emission characteristics of a plurality of devices develop a large variance. A quantitative approach will be discussed in an embodiment set forth below.
Accordingly, in order to prevent a variance is applied device voltage in the forming process, it is necessary that the common wiring connecting a plurality of devices and introducing electric power to them be made wiring having a low resistance. This demand regarding wiring becomes even more important as the number of devices connected to the common wiring increases. This imposes a great limitation upon degree of freedom in terms of manufacturing and designing the electron source and image forming apparatus and in terms of the manufacturing process. One result is an apparatus of high cost.
Problems 2) and 3) mentioned above will now be described in detail.
In the forming process, an electron emission portion is formed in an device by passing electric current. Owing to such electrification, however, power is expended in the common wiring and in the devices and is converted to Joule heat. This is accompanied by a rise in the temperature of the substrate. Meanwhile, a change in form at the formation of the electron emission portion of the device is susceptible to the influence of temperature. Accordingly, a variance and fluctuation in the temperature of the substrate have an influence upon the electron emission characteristic of the device. In particular, in an electron source and image forming apparatus in which a plurality of devices are disposed, an increase in the devices to undergo forming simultaneously is accompanied by a problem even greater than the occurrence of variance owing to the voltage drop in the common-wiring. For example, a distribution is produced in the rising temperature at the central portion of the substrate and at the edges thereof where the heat escapes. The temperature of the central portion rises above that of the edge portions and a variance is produced in the electron emission characteristic. As a result, in a case where an image forming apparatus is manufactured, the variance in the electron emission characteristics of the devices leads to various inconveniences, such as a difference in luminance. This leads to a decline in picture quality.
At the same time, the heat produced subjects the substrate to thermal shock or deformation. This leads to safety-related problems such as rupture in an image forming apparatus constituting an evacuated apparatus in a case where the apparatus makes use of a vessel that must withstand the pressure of the atmosphere.
The following difficulties also arise in addition to the problems described above: